30 INTRODUCTION 



ganglion. The anterior roots of spinal nerves contain 

 what are called efferent nerve fibers, that is, fibers carry- 

 ing impulses from the nerve center. These are sometimes 

 called motor nerve fibers, because their stimulation usually 



results in motion; but the 

 term is not strictly accurate, 

 since other than motor im- 

 pulses may pass over efferent 

 nerves. Posterior roots con- 

 Fig. 21. -Diagram of cross sec- tain afferent nerve fibers, that 

 tion of spinal cord, showing is, fibers carrying impulses 

 nerve roots. toward the nerve center. They 



P posterior root of spinal nerve. 



G ganglion. A anterior root. are also called sensory nerve 

 S spinal nerve. fibers, because when they are 



stimulated feeling or sensation most often results, but other 

 impulses than sensory ones may be conveyed by them. 



27. The Sympathetic System consists of a chain of ganglia 

 lying on each side and in front of the spinal column, of 

 three main plexuses (or nerve networks) in the cavities of 

 the chest and abdomen, of many small ganglia in all parts 

 of the body, and of an immense number of fine nerve fibers 

 (Fig. 22). Each ganglion of the chain is connected by 

 nerve fibers with the one above and the one below, as well 

 as with the spinal cord. In general, the number of pairs 

 of ganglia corresponds to the number of vertebrae, or seg- 

 ments of the backbone; but there are only three pairs of 

 ganglia in the neck, and in front of the coccyx, or last 

 segment of the backbone, there is only a single ganglion. 



28. Gray and White Nervous Matter. Two kinds of 

 nervous matter, easily distinguished by their color, are 

 found in the body. In the cerebrum and the cerebellum 

 the gray matter is mainly in the surface layer, called the 

 cortex, the deeper portions being of white matter ; while in 



